Abstract
During endoplasmic reticulum-associated degradation (ERAD), the cytoplasmic enzyme N-glycanase 1 (NGLY1) is proposed to remove N-glycans from misfolded N-glycoproteins after their retrotranslocation from the ER to the cytosol. We previously reported that NGLY1 regulates Drosophila BMP signaling in a tissue-specific manner (Galeone et al., 2017). Here, we establish the Drosophila Dpp and its mouse ortholog BMP4 as biologically relevant targets of NGLY1 and find, unexpectedly, that NGLY1-mediated deglycosylation of misfolded BMP4 is required for its retrotranslocation. Accumulation of misfolded BMP4 in the ER results in ER stress and prompts the ER recruitment of NGLY1. The ER-associated NGLY1 then deglycosylates misfolded BMP4 molecules to promote their retrotranslocation and proteasomal degradation, thereby allowing properly-folded BMP4 molecules to proceed through the secretory pathway and activate signaling in other cells. Our study redefines the role of NGLY1 during ERAD and suggests that impaired BMP4 signaling might underlie some of the NGLY1 deficiency patient phenotypes.
Highlights
N-Glycanase 1 (NGLY1; known as peptide:N-glycanase or PNGase) is a cytoplasmic enzyme capable of removing N-glycans from glycoproteins (Suzuki et al, 2002)
To provide further evidence for ER stress in Ngly1–/– cells especially upon BMP4 overexpression, we examined the levels of two additional ER stress markers: phosphorylated IRE1a, which is an indicator of unfolded protein response activation (Zhang and Kaufman, 2004; Korennykh et al, 2009), and OS9, an ER lectin which is upregulated upon ER stress, selectively binds misfolded glycoproteins, and facilitates their transport to the retrotranslocation machinery (Kim et al, 2005; Alcock and Swanton, 2009; Satoh et al, 2010)
To examine whether NFE2L1 deglycosylation is mediated by the ER-associated NGLY1 similar to BMP4, we examined the ability of wildtype and valosin containing protein (VCP)-binding-deficient mutant versions of human NGLY1 to restore the proteasomal bounce-back response in Ngly1–/– mouse embryonic fibroblasts (MEFs)
Summary
N-Glycanase 1 (NGLY1; known as peptide:N-glycanase or PNGase) is a cytoplasmic enzyme capable of removing N-glycans from glycoproteins (Suzuki et al, 2002). NGLY1 and its homologs recognize and cleave Nglycans from their target proteins, changing the asparagine (N) to aspartic acid (D) upon removing the sugar chain (Hirayama et al, 2015). Proteins that fail to fold properly are recognized, retrotranslocated from the ER into the cytosol and undergo proteasomal degradation through a process called ER-associated degradation (ERAD) (Smith et al, 2011; Brodsky, 2012). The yeast NGLY1 homolog (PNG1) is part of the ERAD mechanism (Hirayama et al, 2015), and NGLY1 is proposed to contribute to ERAD by removing N-glycans from misfolded proteins after their retrotranslocation, thereby promoting their degradation
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